1. Field of the Invention
The present invention relates to furnaces, and in particular to control of furnace temperatures.
2. Brief Description of the Related Art
An increased demand for flat glass (produced in float furnaces) all over the world is expected to become the major driving force for oxidant-fuel (xe2x80x9coxyxe2x80x9d) boosting technology. Joshi, M. L., et al, xe2x80x9cOxygen-fuel boosting as applied to float glass furnacesxe2x80x9d, Presented at the American Flame Research Committee, 1996, AFRC Spring Members Technical Meeting, Orlando, Fla., May 6-7, 1996. Due to relatively long engineering and construction phases for green field float plants, an effective on-the-fly oxy-boosting solution to meet immediate market needs is considered to be both a cost-effective and low-risk option by those in industry.
Typical float furnaces are side-fired regenerative types with five to eight ports per side. FIG. 1 illustrates a typical float glass tank 10 with six ports 22. The furnace 10 includes a sidewall 12, an interior chamber or space 14, and an entrance or doghouse 16. A waist section 18 receives the glass flow downstream of the interior chamber 14, from which the glass moves to a conditioning end 20 of the furnace. Due to the large dimensions of the tank, only cross firing is possible. FIG. 1 illustrates a six-port furnace with one regenerator chamber 24 assigned for each port. The regenerator chambers are used for preheating combustion air to between about 2200xc2x0 F. (1204xc2x0 C.) and about 2400xc2x0 F. (1316 xc2x0C.). A 20 to 30 minute cyclic process for heat recovery is typically applied using the exhaust gases. Air-fuel burners (not illustrated) are installed on each port with 2 to 3 burners per port. The burners are fired xe2x80x9cunder port,xe2x80x9d xe2x80x9cthrough port,xe2x80x9d or using xe2x80x9cside of portxe2x80x9d firing configuration.
The furnace is also provided with oxy-boost burners 26. The common oxy-boost system is xe2x80x9cport 0xe2x80x9d boosting, meaning that the oxy-boost burners 26 are positioned between the charging wall at doghouse 16 and the first port 22. Typically, standard oxy burners or high performance staged oxy burners are installed at the port 0 location. The oxy-boost firing capacity can be as much as 5% to 20% of the total melter firing capacity. The oxy-boost process is used to attempt to increase the furnace pull rate, increase glass quality (e.g., reduced number of seeds, stones, etc., per ton of glass) at the same pull rate or at higher pull rate, decrease or maintain furnace peak refractory temperatures at the same or higher pull rate, decrease or maintain regenerator chamber temperatures at the same or higher pull rate, avoid plugging problems in the regenerator, as well as overcoming other difficulties which can not be achieved by the air-fuel firing of the port burners alone.
The challenge to successful use of oxy-boost technology is optimum firing of oxy burners at port 0 (or other strategic locations), coupled with specific (measured) changes in the air-fuel firing rate of the port burners. Thus, prior systems have attempted to optimize the overall heat-input profile to yield the above benefits.
The furnace operators at flat glass plants have attempted this optimization process by trial and error methods. For example, a human operator may reduce the air-fuel firing rate incrementally for each port, and subsequently increase the oxy-boost firing rate until the desired furnace refractory temperatures and profile, desired glass bottom temperatures and profile, required pull rate, and target glass quality numbers are obtained.
The difficulty in prior furnace operations is compounded due to the fact that oxy-boost control systems are not the same as, integrated with, or communicate with the air-fuel combustion control. Thus, retrofit oxy-boost systems are generally a stand-alone type and must be operated separately to manage the overall furnace operation. In many instances, the changes in oxyboost firing and air-fuel firing are not complementary and can upset the furnace operation. This can result in poor product quality or possible overheating of critical furnace refractory during this period.
The set point adjustments to achieve the desired furnace crown temperatures and glass bottom temperatures during oxy-boosting can take several weeks to several months, depending on the furnace and expertise level of the operator. A longer furnace settling time means greater loss in furnace productivity, poor product quality, and higher operating costs. Furthermore, if the furnace pull rate is changed for some reasons (say, due to changes in market demand), the whole adjustment procedure has to be repeated and new set points have to be determined for both oxy-boosting and air-fuel firing.
According to a first exemplary embodiment, a system useful for heating a product comprises a furnace having a sidewall and an interior space, at least one oxy-fuel burner positioned to direct a flame into the furnace interior space, a source of an oxidant in fluid communication with the at least one oxy-fuel burner, a source of fuel in fluid communication with the at least one oxy-fuel burner, a first valve set interposed between the oxy-fuel burner and the sources of fuel and oxidant, the first valve set operable to control the flow of oxidant and fuel to the at least one oxy-fuel burner, at least one air-fuel burner positioned to direct a flame into the furnace interior space, a source of air in fluid communication with the at least one air-fuel burner, a source of fuel in fluid communication with the at least one air-fuel burner, a second valve set interposed between the air-fuel burner and the sources of fuel and air, the second valve set operable to control the flow of air and fuel to the at least one air-fuel burner, at least one furnace condition input device generating at least one output signal, and a controller in communication with the at least one furnace condition input device to receive the at least one furnace condition input device output signal, the controller having a setpoint value for the at least one furnace value, the controller generating at least one control signal based on a comparison of the output signal and the setpoint, the controller being in communication with the first valve set to communicate the at least one control signal to the first valve set to set the flow rates of oxidant and fuel through the first valve set and to the oxy-fuel burner.
According to a second exemplary embodiment, a process of operating a furnace comprises the steps of firing an oxy-fuel burner into the furnace to heat a load, firing an air-fuel burner into the furnace to heat the load, measuring a furnace process parameter, inputting the measured process parameter into a controller, the controller having at least one setpoint for the measured process parameter, and controlling both the firing of the oxy-burner and the firing of the air-fuel burner with the controller based on a comparison of the measured process parameter and the at least one setpoint value.
Still other objects, features, and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings.